It is well known that the SP-2 monoclonal antibody (IgG1 subclass) reacts with an antigen of 90,000 dalton molecular weight, not yet fully characterized and designated as 90 KD. This antigen has a cytoplasmic localization and is found tumoral tissue and in the sera of patients suffering from breast cancer and other neoplastic diseases (S. Iacobelli et al.; Cancer Research 46, 3005-10, 1986; Anticancer Research 8, 761-4, 1988; Breast Cancer Research and Treatment 11, 19-30, 1988; Gynecologic Oncology, 35, 286-9, 1989).
Now, it has been surprisingly found that the 90 KD antigen is also present in the sera of patients infected with HIV. Additionally, it has been found that the positivity percentage and the average antigen concentrations in serum are proportional to the extent of the infection, with increasing values on passing from the HIV-seropositivity conditions in asymptomatic subjects to the ARC (AIDS related complex) and to the AIDS (acquired immunodeficiency syndrome). Furthermore 90 KD antigen levels higher than those in normal subjects are found in the serum of anti-HIV seronegative subjects at high risk of HIV infection. The assay of the 90K antigen is performed by utilizing the monoclonal antibody SP-2, purified from the correspondent hybridoma cell line, deposited at DSM DEUTSCHE SAMMLUNG VON MIKROORGANISMEN UND ZELLKULTUREN GmbH under the Budapest Treaty under the accession number DSM ACC2116, Mascheroder Weg 1 B D-3300 Braunschweig, Germany, and at C.N.C.M. the Collection Nationale de Cultures de Microorganismes at the Pasteur Institute, Paris, France, under the accession number I-1083, as previously described by the present inventor, Iacobelli et al., 1986, Cancer Research, 46, 3005-3010 and Iacobelli et al., 1988, Breast Cancer Research and Treatment, 11, 19-30. The assay thus represents a very useful support for the early diagnosis of the HIV infection just before the patient shows the symptoms of the disease and for the monitoring of the progress of the infection itself. Accordingly, the test can be employed for selecting the patients who are to be treated therapeutically, and then for evaluating the efficacy of the treatment itself.